Do Black Plate 6L6 Tubes Cause Red Plating in Mac Amps?

I'm hoping someone can get me the correct information regarding whether black plate 6L6GCs by any manufacturer (but especially RCA) can cause red plating in Mac amps (no other cause other than the tube having a black plate)? The friend who told me this said that there is no explanation for it, but, because of the red plate danger, I should always avoid using black plate 6L6 tubes in Mac amps.

I know that McIntosh-labeled tubes were mostly black plate, so this puzzles me. I also searched the internet and could not find anything on this unexplained phenomenon.

Has anyone else heard of this (or experienced it) or it just some unsubstantiated rumor/urban legend floating around?

"McIntosh amps will correctly perform with 6L6GC tubes. The earlier 6L6 tubes do not have the voltage rating to work in these amps. Red platting is not an issue using 6L6GC tubes as long as the amp is in proper operational condition."

Standard 6L6 output tubes do not have a glass envelope, it is metal. The 6L6G has a glass envelope as does the 6L6GC. The 6L6 tube envelope is metal so no way can you see the plates or even tell (until it's too late) if red plating occurred internally. As McIntosh states, 6L6 tubes (or 6L6G) were not rated for higher plate voltages like the 6L6GC and are not recommend for use in an MC30 or MC240. The MC30 did come with 1614 output tubes which had a metal envelope but they were rated for higher plate voltages than standard 6L6 metal tubes. I think your friend was referring to RCA 6L6GC glass tubes which did have black plates internally. I have never heard that they were more susceptible to red plating than any other tubes. In fact, they are very highly regarded and NOS samples go for big bucks today. Plus you don't have to use matched output tubes in the old MC series of tube amps. They were designed to compensate for mismatched output tubes due to the output transformer design, I believe. However, use unmatched output tubes in a Dynaco ST70 and you probably will see red plating as one tube robs the other of current and goes into thermal runaway. That I have seen happen myself.

I think you are spot on. Perhaps the rumor began with someone having a red plate experience with non-GC black plate tubes and then assumed that all black plate tubes would red plate. My friend specifically said that RCA black plates were bad, but I have some McIntosh-branded black plate tubes that were made by RCA. I wouldn't think that McIntosh would sell tubes that would unexplainably red plate; that would be a PR nightmare in the audiophile world.

I think you are spot on. Perhaps the rumor began with someone having a red plate experience with non-GC black plate tubes and then assumed that all black plate tubes would red plate. My friend specifically said that RCA black plates were bad, but I have some McIntosh-branded black plate tubes that were made by RCA. I wouldn't think that McIntosh would sell tubes that would unexplainably red plate; that would be a PR nightmare in the audiophile world.

Click to expand...

I volunteer to assume the risk of using RCA black plate 6L6GC's that anyone may want to dispose of...

Usually caused by incorrect or loss of control grid (G1) bias, the outer part of the innards of an output tube glows an abnormal dull to bright red. If allowed to continue, will result in the destruction of the tube and possibly associated components.

In some examples, an output tube may spontaneously red-plate from a runaway bias cancelling cycling, caused by cathode material being sputtered onto the control grid by being overstressed. In this condition, the tube is defective and must be replaced.

6L6, 6L6G, 6L6GA, and 6L6GB are all glass bottle early versions that will not work (for very long!) in a Mc amp as they have a Max plate watts of 19, Max plate volts of 360 and Max screen volts of 270 @ 2.5 amps.
6L6GB is Max plate watts of 30, Max plate Volts of 500, Max screen volts 450 @ 5 amps.